This invention relates to a method and apparatus for measuring pulse wave transmission, and more particularly pulse transit time, of a human or mammalian subject.
The human (or mammalian) pulse is a traveling wave disturbance that emanates from the heart and travels throughout the arterial system. Since the velocity of pulse propagation in a liquid is directly proportional to the pressure of the liquid, it is possible to detect blood pressure by measuring the propagation velocity of the pulse wave. The propagation velocity of the pulse wave can be measured by detecting the pulse transit time, which is the time period required for the pulse wave to travel between two spaced arterial pulse points.
An example of a blood pressure monitoring system that utilizes pulse transit time can be found in U.S. Pat. No. 4,245,648 to Trimmer et al. This system includes a pair of piezoelectric sensors closely spaced (by about 3 cm.) along the brachial artery to detect the traveling pulse wave. Pulse transit time is determined as the difference between arrival times of the pulse wave at the two sensors.
The use of piezoelectric sensors as described in the aforementioned patent leads to several significant practical limitations. For example, piezoelectric sensors commonly exhibit limited sensitivity at frequencies below about 2 Hz. The pulse rate of a human adult is ordinarily around 60 beats per minute, or 1 Hz. The pulse rate of a human infant is typically about 120 to 180 beats per minute, or 2 to 3 Hz. Thus, the practical requirements of a system using piezoelectric sensors for monitoring human subjects may push the limit of, or even exceed, the performance capabilities of the sensors. Another practical limitation stems from the fact that piezoelectric sensors require the presence of electrically conductive material (e.g., electrodes and lead wires) at the sensor location on the test subject. The system consequently cannot be used in environments where the presence of such materials would be problematical. For example, electrically conductive materials have been known to cause severe burning of patients undergoing MRI examinations, due to the presence of strong radio frequency fields generated by the MRI machine. Still another limitation is imposed by the location of the sensors in mutual proximity along the same artery. Locating the sensors in mutual proximity means that the pulse transit time to be measured will be very short and inherently more difficult to measure accurately. It will be appreciated that a given amount of error becomes more significant as the time period being measured becomes shorter.
In one of its aspects, the present invention provides a method of measuring pulse transit time that is especially useful (although not limited to use) with pulse sensors located at substantially spaced pulse points. For example, one of the sensors may be located over the brachial artery near or on the upper arm, and the other sensor located over the radial artery on the wrist. The method involves differentiation of the respective pulse wave signals from the sensors to determine corresponding points of the two signals, such as the points of maximum slope. The time delay between these points is then determined, thus yielding the pulse transit time. Differentiating the two pulse wave signals facilitates the identification of corresponding points of the signals, even though the pulse waveforms may differ somewhat when the sensors are substantially spaced from one another as noted above. Further, it allows for the selection of a consistent time marker (e.g., point of maximum slope) upon which to base the pulse transit time calculation from one pulse wave to the next. This is particularly advantageous since the pulse waveform ordinarily varies from one heartbeat to the next.
In another of its aspects, the invention provides an apparatus for implementing the foregoing method. The apparatus includes a pair of pulse sensors and a signal processing unit that processes the respective pulse wave signals of the pulse sensors in accordance with the method.
In another of its aspects, the present invention provides an apparatus for measuring pulse transit time including at least one pulse sensor, and preferably two pulse sensors, constituted by a variable coupler fiberoptic sensor having an improved design to be described herein. The apparatus further includes a signal processor and may be used to implement the aforementioned method or to implement other methods of measuring pulse transit time.